In rock drilling, a drilling tool which is connected to a rock drilling device by one or more drill string components is often used. The drilling can be carried out in a number of different ways, in which a commonly occurring method is percussion drilling, in which a pulse generating device, a percussion device, is used to generate percussions with the aid of a reciprocating piston. The percussion piston strikes the drill string, usually via a drill shank, so as, by transfer of kinetic energy to the drill string, to produce shock waves, which are propagated through the drill string to the drilling tool and then onward from the tool to the rock for release of energy of the shock wave.
The percussion piston is typically driven by pressurization and depressurization of drive surfaces acting upon the percussion piston in the longitudinal direction of the drill string, the said pressurization usually being realized with the aid of hydraulically and/or pneumatically working means.
Pulse generating devices of this kind, in which the shock wave is generated by transfer of the kinetic energy of the percussion piston to the drill shank/the drill string, can give rise however, at least under certain operating conditions, to undesirable side effects, such as that the kinetic energy generated with the reciprocating motion of the percussion piston can produce an undesirable negative effect upon the pulse generating device and/or drill string and/or tool.
There is also another type of pulse generating devices, in which the shock wave energy, instead of being generated, as above, by means of released kinetic energy from a reciprocating piston, is instead generated by the release of stored elastic energy, which is transferred to the drill string from an impact means and/or an energy store via the impact means, which in this case only performs a very small movement, i.e. the kinetic energy which is transferred is substantially lower than the transferred elastic energy.
According to the prior art, such solutions generate shock waves with lower energy compared with a conventional percussion piston in which, in order to maintain the effectiveness of the drilling, the lower shock wave energy is compensated for by higher-frequency generation of the shock waves.
One problem with such pulse generating devices is, however, that the substantially higher shock wave frequency which is required to obtain the desired drilling effect places demands, in turn, upon the mechanism that opens and closes ducts to the drive surfaces which act upon the impact means in the generation of the said shock waves.
In WO2004/073933, an example is shown of a pulse generating device of this kind, in which a rotary control valve is used to achieve rapid opening and closure of ducts to a drive surface acting upon the impact means. The shown solution has the drawback, however, that a drive motor is required to drive the control valve, and this drive motor entails that the pulse generating device acquires a larger diameter due to the diameter of the drive motor. This is aggravated, moreover, by the fact that, especially where a high rotation frequency is desired, the drive motor must have a certain diameter to prevent the rotation speed difference between the valve and the motor from becoming too large, since a large difference can result in the desired drive motor speed (valve speed) not being reached for design reasons.
In tunnelling, for example, the desired drilling machine diameter is a major drawback, since a large drilling machine diameter entails an unnecessarily large quantity of material having to be removed from the mine to allow a constant diameter to be maintained through the tunnel. The larger quantity of removed material also means that a greater volume has to be refilled with concrete, for example, following drilling.
There is therefore a need for an improved drive mechanism for drilling machines intended for high-frequency operation.